EP0497675A1 - Process for manufacturing a fine dendritic cadmium powder and powder obtained by this process - Google Patents

Process for manufacturing a fine dendritic cadmium powder and powder obtained by this process Download PDF

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Publication number
EP0497675A1
EP0497675A1 EP92400200A EP92400200A EP0497675A1 EP 0497675 A1 EP0497675 A1 EP 0497675A1 EP 92400200 A EP92400200 A EP 92400200A EP 92400200 A EP92400200 A EP 92400200A EP 0497675 A1 EP0497675 A1 EP 0497675A1
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cadmium
powder
sponge
approximately
dendrites
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German (de)
French (fr)
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EP0497675B1 (en
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Luc Albert
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Metaleurop SA
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Metaleurop SA
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C5/00—Electrolytic production, recovery or refining of metal powders or porous metal masses
    • C25C5/02—Electrolytic production, recovery or refining of metal powders or porous metal masses from solutions

Definitions

  • the present invention relates to a process for the preparation of a dendritic cadmium powder, as well as a powder obtained by the process.
  • the electrode structures are particularly studied in order to be able to contain a charge of active material as high as possible (Number of amperes.hourest hour) and as available as possible (maximum intensity).
  • the negative electrode structure usually made of sintered nickel
  • PBT structure in which a mixture of cadmium oxide and metal powder is coated on a strip.
  • the role of the conductive powder is to distribute the current of electrons homogeneously in the volume of the active mass of cadmium hydroxide.
  • Japanese patent application published under No. 55-76569 on 9.6.80 teaches the preparation of such a powder and its incorporation into the electrode in such proportions.
  • the present invention is based on the observation that, with a powder urn which deviates from the spherical shape (in other words with a form factor which is much greater than 1), and in particular with a dendritic powder, the electrical performance is greatly improved, and in particular the volume energy is increased.
  • a dendritic powder is added to the paste of the electrode in a quantity substantially lower than a spherical powder, resulting in a gain of weight.
  • Document FR-A-2 194 792 teaches a process for the preparation of a porous electrode from a cadmium powder of acicular or dendritic nature. This powder, obtained by deposition on an electrode and then dry scraping, is then compressed to form the electrode, and is not intended to be used as a current distribution agent as mentioned above. More specifically, the operating conditions described in this document are such that the powder does not have the required fineness. In addition, the principle of electrolysis described in this patent oblique to work with small amounts of electricity, with extremely frequent scraping.
  • the present invention thus aims to propose an electrolytic process for obtaining a cadmium dendritic powder, which makes it possible to obtain, by appropriate control of simple parameters, a powder of quality and of particularly suitable characteristics, in particular in terms of fineness, for incorporation into a negative nickel / cadmium accumulator electrode. It also proposes a process that can be implemented with large amounts of electricity, to obtain before powder reduction an electrode thickness of up to several centimeters without suffering from homogeneity defects.
  • the invention also relates to a dendritic powder obtained by the above method, characterized in that its particles have the form of ferns comprising a central core from which secondary oblique dendrites leave obliquely.
  • An advantage of the method according to the invention resides in that it offers the possibility of constituting a metallic matrix whose physical characteristics can be chosen without having to undergo the strong constraint of the particle size distribution.
  • the inventors were able to determine the conditions for obtaining, by electrolytic deposition, a dendritic structure or else a sponge structure.
  • the transition from one form of structure to another follows from the mode of crystallization.
  • the dendritic structure evolves towards the sponge when the section of the crystals oriented in the field decreases and that the two-dimensional germination continues on the pre-existing dendrites.
  • the constitution of the sponge by deposition on the cathode does not pose any particular starting difficulty.
  • Substrates can be used, for example stainless steel or titanium. It does not matter whether the surface is virgin or whether there remains a cadmium residue from the previous operation.
  • the separation of the sponge from its substrate is carried out by light mechanical means of conventional type.
  • the separated sponge is then washed in order to recover the electrolyte which still permeates it.
  • the special structure of the sponge allows very effective washing with a very small amount of water.
  • the sponge proves to be perfectly chemically stable, either with respect to dissolution by acid attack or by air oxidation.
  • the second operation of the process consists in performing a shredding of the sponge. It is carried out in tank disintegration devices provided with specific stirring mobiles, operating continuously or discontinuously. In order to promote a complete release of particles of the sponge, it is preferable here to work with a pulp rate that does not exceed 200 g of dry matter per liter. As will be seen below, the peripheral speed of the agitation mobile is an important factor in obtaining an appropriate particle size.
  • a sieving operation intended to remove coarse particles, and preferably particles larger than 125 ⁇ m, is advantageously provided, but optionally.
  • the disintegration stage releases particles whose size and solidity are determined essentially by the operating conditions of the electrolytic stage of constitution of the sponge, and are only very weakly influenced by a too long residence time of the material in the disintegration apparatus and by the choice of the geometry of the stirring mobiles.
  • shredding machines have therefore been tested, with or without counter-blades, without significantly modifying the morphology and particle size of the powder. In addition, no overgrinding of the powder was observed.
  • the good mechanical strength of the particles constituting the pulp allows storage in the decanted state without modifying the particle size distribution.
  • the pulp obtained at the end of the disintegration can be pumped, for example by a centrifugal vortex pump, without undergoing any particle size alteration.
  • the morphology of the powder obtained is characteristic of the process according to the invention.
  • the particles have the form of ferns made up of a central column from which leave, with an angle of the order of 60 °, secondary ferns.
  • the overall shape is generally acicular, a shape well suited to the intended application.
  • the electrolysis cell can be supplied either with a pure cadmium solution or with metallic cadmium of appropriate purity.
  • a concentrated solution is preferably chosen.
  • the associated anion is advantageously sulfate.
  • the acidity of the solution can vary for example between 5 and 80 g / l of sulfuric acid.
  • the total content of metallic impurities in the solution, expressed relative to cadmium, must be less than 100 g / t.
  • the electrolysis cell is powered by cadmium metal
  • it can take any suitable form, preferably with a purity of 99.99% or better.
  • the electrolyte is composed of cadmium sulfate and sulfuric acid.
  • the acid content is conditioned by the search for a good ionic conductivity of the electrolyte. This content is advantageously between 5 and 100 g / l, a value close to 50 g / l being particularly advantageous because it gives a very good conductivity while limiting the acid corrosion of the sponge.
  • the choice of the cadmium concentration is closely linked to the choice of the cathodic current density. Tests carried out by the Applicant have made it possible to discover that, in the range of current densities ranging from approximately 700 to approximately 1500 A / m2, it is advantageous for the cadmium concentration to respond to the following relationship: 100 Am / kg ⁇ J / (CD) ⁇ 200 Am / kp or J is the current density expressed in A / m2; (CD) is the cadmium concentration expressed in kg / m3.
  • the cadmium concentration is preferably between 4 and 15 g / l, more preferably between 7 and 11 g / l.
  • the operating temperature is preferably maintained in a range between 20 and 35 ° C, more preferably between 25 and 30 ° C.
  • the cathode substrate is preferably stainless steel or titanium. It was found that good adhesion of the sponge was obtained with a surface roughness corresponding to the raw rolling state.
  • the circulation of the electrolyte is ensured either naturally when the oxygen is released, for a cell with insoluble anodes, or in a provoked way.
  • the choice of circulation type has practically no influence on the morphology of the sponge.
  • the duration of electrolysis between two debates is preferably between 4 and 8 hours.
  • a duration of the order of 6 hours is particularly suitable.
  • the concrete design of the electrolysis cells is of the conventional type, and will not be described in detail. It is possible, for example, to use cells of the type used in the zinc or copper industry.
  • the composition of the electrolyte does not remain stable. Indeed, the reaction at the cathode, where the protons are reduced and hydrogen is generated, constitutes a parasitic reaction which causes a decrease in the acidity of the medium, with which is associated an increase in the cadmium concentration.
  • the electrolysis process with soluble anodes with an electrolysis process with insoluble anodes, working on the same electrolyte.
  • this surface constitutes a determined percentage of the total cathodic surface, percentage equal to the faradic cathodic yield of hydrogen release, the excess of aforementioned anodic dissolution.
  • the acidity consumed by the above-mentioned parasitic reaction is also generated on the insoluble anodes.
  • the system is therefore generally balanced and can operate under stable conditions practically without the need for addition or purging, which guarantees constant quality for the sponge formed, and consequently for the powder.
  • the sponge After electrolysis then deburring and washing of the sponge as indicated above, the sponge is subjected to the disintegration operation.
  • the shredding action is carried out by a stirring mobile having no significant pumping or shearing function. We are mainly looking for a shock effect on the peripheral parts of the mobile which have a small active surface.
  • the essential parameter is the peripheral speed of the mobile. It is preferably situated between 20 and 50 m / s for diameters of stirring mobiles varying between 83 and 380 mm. For speeds below this range, there is a rapid increase in the rate of particles refused to sieving. Concretely, it was found that, for a mobile with a diameter of 380 mm, a peripheral speed of 30 m / s was sufficient to reach a rejection rate at sieving at 125 ⁇ m of less than 0.5%.
  • the residence time of the sponges in the disintegration apparatus is for example between 3 and 5 minutes. However, it has been found that an excess of residence time of 100 to 200% over these durations has no consequence on the particle size distribution.
  • the pulp rate is fixed at a value compatible both with the productivity requirements of the process and with the conservation requirement of the particle size distribution. Concretely, an amount of dry matter per liter of pulping solution between 50 and 200 g / l is suitable. Beyond the upper limit, the particle size distribution becomes coarser.
  • the pulp is sieved, as indicated above, for example using a vibrating sieve.
  • the pulp is then decanted and conditioned. An oxidation rate of less than 1% per month has been observed under wet storage conditions.
  • the process of the invention guarantees a very high titer in metallic cadmium compared to the total cadmium.
  • the final product is very little oxidized.

Abstract

Process for obtaining a fine powder of dendritic cadmium, characterised in that it comprises the following stages: (a) electrolytic production of cadmium metal on an electrode under conditions such that a sponge consisting of entangled polymorphous dendrites is formed, (b) removing the sponge from the frame and washing it, (c) disintegrating the sponge in a pulpous medium under such conditions that the dendrites are released to obtain a dendritic powder of particle size a

Description

La présente invention concerne un procédé d'élaboration d'une poudre dendritique de cadmium, ainsi qu'une poudre obtenue par le procédé.The present invention relates to a process for the preparation of a dendritic cadmium powder, as well as a powder obtained by the process.

Dans le cadre général du développement des accumulateurs nickel/cadmium, on tend constamment à rechercher une amélioration des performances. Ainsi les structures d'électrodes sont particulièrement étudiées afin de pouvoir contenir une charge de matière active la plus élevée possible (Nombre d'Ampères.heure maximal) et la plus disponible possible (intensité maximale).Within the general framework of the development of nickel / cadmium accumulators, there is a constant tendency to seek an improvement in performance. Thus the electrode structures are particularly studied in order to be able to contain a charge of active material as high as possible (Number of amperes.hourest hour) and as available as possible (maximum intensity).

Un aspect particulier de ces recherches a consisté à élaborer de nouvelles techniques de fabrication d'électrodes qui mettent en oeuvre des quantités plus faibles de ces matières.A particular aspect of this research consisted in developing new techniques for manufacturing electrodes which use smaller quantities of these materials.

Ainsi, notamment dans le domaine des accumulateurs portables, la structure d'électrode négative, habituellement en nickel fritté, a été remplacée par une structure dite PBT dans laquelle un mélange d'oxyde de cadmium et de poudre métallique est enduit sur une bande. Dans cette technique connue, le rôle de la poudre conductrice est de distribuer de façon homogène le courant d'électrons dans le volume de la masse active d'hydroxyde de cadmium.Thus, in particular in the field of portable accumulators, the negative electrode structure, usually made of sintered nickel, has been replaced by a so-called PBT structure in which a mixture of cadmium oxide and metal powder is coated on a strip. In this known technique, the role of the conductive powder is to distribute the current of electrons homogeneously in the volume of the active mass of cadmium hydroxide.

On utilise classiquement un certain nombre de types de poudres métalliques, soit de cadmium, soit de nickel. Souvent sphériques ou sphéroïdes, ces poudres connues sont ajoutées en des proportions importantes, typiquement de l'ordre de 20% en poids, pour atteindre la résistivité requise pour l'électrode.Conventionally, a certain number of types of metallic powders are used, either cadmium or nickel. Often spherical or spheroid, these known powders are added in significant proportions, typically of the order of 20% by weight, to achieve the resistivity required for the electrode.

La demande de brevet japonais publiée sous le No. 55-76569 le 9.6.80 enseigne l'élaboration d'une telle poudre et son incorporation dans l'électrode dans de telles proportions.Japanese patent application published under No. 55-76569 on 9.6.80 teaches the preparation of such a powder and its incorporation into the electrode in such proportions.

La présente invention est basée sur la constatation suivant laquelle, avec urne poudre qui s'écarte de la forme sphérique (en d'autres termes avec un facteur de forme qui est très supérieur à 1), et en particulier avec une poudre dendritique, les performances électriques sont grandement améliorées, et en particulier l'énergie volumique est accrue. Cela signifie que, pour obtenir les mêmes performances qu'avec une électrode de l'art antérieur, une poudre dendritique est ajoutée à la pâte de l'électrode en une quantité sensiblement plus faible qu'une poudre sphérique, d'où un gain de poids.The present invention is based on the observation that, with a powder urn which deviates from the spherical shape (in other words with a form factor which is much greater than 1), and in particular with a dendritic powder, the electrical performance is greatly improved, and in particular the volume energy is increased. This means that, to obtain the same performance as with an electrode of the prior art, a dendritic powder is added to the paste of the electrode in a quantity substantially lower than a spherical powder, resulting in a gain of weight.

Le document FR-A-2 194 792 enseigne un procédé d'élaboration d'une électrode poreuse à partir d'une poudre de cadmium de nature aciculaire ou dendritique. Cette poudre, obtenue par dépôt sur une électrode puis grattage à sec, est ensuite comprimée pour former l'électrode, et n'a aucune vocation à être utilisée comme agent de répartition du courant comme évoqué ci-dessus. Plus précisément, les conditions opératoires décrites dans ce documents sont telles que la poudre ne présente pas la finesse requise. En outre, le principe d'électrolyse décrit dans ce brevet oblique à travailler avec des quantités d'électricité faibles, avec des grattages extrêmements fréquents.Document FR-A-2 194 792 teaches a process for the preparation of a porous electrode from a cadmium powder of acicular or dendritic nature. This powder, obtained by deposition on an electrode and then dry scraping, is then compressed to form the electrode, and is not intended to be used as a current distribution agent as mentioned above. More specifically, the operating conditions described in this document are such that the powder does not have the required fineness. In addition, the principle of electrolysis described in this patent oblique to work with small amounts of electricity, with extremely frequent scraping.

La présente invention vise ainsi à proposer un procédé électrolytique d'obtention d'une poudre dendritique de cadmium, qui permette d'obtenir par un contrôle approprié de paramètres simples un poudre de qualité et de caractéristiques particulièrement appropriées, notamment en matière de finesse, pour une incorporation dans une électrode négative d'accumulateur' nickel/cadmium. Elle propose également un procédé pouvant être mis en oeuvre avec des quantités d'électricité importantes, pour obtenir avant réduction en poudre une épaisseur d'électrode pouvant atteindre plusieurs centimètres sans souffrir de défauts d'homogénéité.The present invention thus aims to propose an electrolytic process for obtaining a cadmium dendritic powder, which makes it possible to obtain, by appropriate control of simple parameters, a powder of quality and of particularly suitable characteristics, in particular in terms of fineness, for incorporation into a negative nickel / cadmium accumulator electrode. It also proposes a process that can be implemented with large amounts of electricity, to obtain before powder reduction an electrode thickness of up to several centimeters without suffering from homogeneity defects.

Elle concerne à cet effet un procédé d'obtention d'une poudre fine de cadmium dendritique, caractérisé en ce qu'il comprend les étapes suivantes :

  • (a) production électrolytique de cadmium métal sur une électrode, dans des conditions telles qu'on forme une éponge constituée de dendrites polymorphes enchevêtrées,
  • (b) débatissage et lavage de l'éponge,
  • (c) délitage de l'éponge en milieu pulpeux dans des conditions telles qu'on libère les dendrites, pour obtenir une poudre dendritique de granulométrie essentiellement inférieure à une limite déterminée.
To this end, it relates to a process for obtaining a fine powder of dendritic cadmium, characterized in that it comprises the following stages:
  • (a) electrolytic production of cadmium metal on an electrode, under conditions such that a sponge made up of entangled polymorphic dendrites is formed,
  • (b) stripping and washing the sponge,
  • (c) disintegration of the sponge in pulpy medium under conditions such that the dendrites are released, in order to obtain a dendritic powder with a particle size essentially below a given limit.

L'invention concerne également une poudre dendritique obtenue par le procédé ci-dessus, caractérisée en ce que ses particules présentent la forme de fougères comportant une âme centrale de laquelle partent en oblique des dendrites secondaires.The invention also relates to a dendritic powder obtained by the above method, characterized in that its particles have the form of ferns comprising a central core from which secondary oblique dendrites leave obliquely.

Un avantage du procédé selon l'invention réside en ce qu'il offre la possibilité de constituer une matrice métallique dont les caractéristiques physiques peuvent être choisies sans avoir à subir la contrainte forte de la distribution granulométrique. Ainsi les inventeurs ont pu déterminer les conditions d'obtention par dépôt électrolytique d'une structure dendritique ou bien d'une structure en éponge. La transition d'une forme de structure à l'autre découle du mode de cristallisation. Ainsi la structure dendritique évolue vers l'éponge lorsque la section des cristaux orientés dans le champ diminue et que la germination bidimensionnelle se poursuit sur les dendrites pré-existantes.An advantage of the method according to the invention resides in that it offers the possibility of constituting a metallic matrix whose physical characteristics can be chosen without having to undergo the strong constraint of the particle size distribution. Thus the inventors were able to determine the conditions for obtaining, by electrolytic deposition, a dendritic structure or else a sponge structure. The transition from one form of structure to another follows from the mode of crystallization. Thus the dendritic structure evolves towards the sponge when the section of the crystals oriented in the field decreases and that the two-dimensional germination continues on the pre-existing dendrites.

Il se crée ainsi, par enchevêtrement, des dendrites polymorphes constituant une véritable structure poreuse qui se caractérise par une densité volumique en cadmium très faible (de l'ordre de 0,1 kg/dm³). On a également observé que les conditions de génération de l'éponge pouvaient être réglées pour obtenir un rendement faradique élevé, typiquement supérieur à 70%, en rendant le procédé particulièrement économique sur le plan énergétique.This creates, by entanglement, polymorphic dendrites constituting a true porous structure which is characterized by a very low cadmium density (of the order of 0.1 kg / dm³). It has also been observed that the conditions for generating the sponge can be adjusted to obtain a high faradaic yield, typically greater than 70%, by making the particularly energy-efficient process.

La constitution de l'éponge par dépôt sur la cathode ne pose pas de difficulté de démarrage particulière. On peut utiliser des substrats par exemple en acier inoxydable ou en titane. Il est indifférent que la surface soit vierge ou qu'il y subsiste un résidu de cadmium de l'opération précédente.The constitution of the sponge by deposition on the cathode does not pose any particular starting difficulty. Substrates can be used, for example stainless steel or titanium. It does not matter whether the surface is virgin or whether there remains a cadmium residue from the previous operation.

Grâce au procédé selon l'invention, on peut réaliser des éponges de cadmium d'épaisseurs très importantes (typiquement de 3 à 6 cm) qui se caractérisent par:

  • une bonne homogénéité de structure sur toute l'épaisseur;
  • un développement en épaisseur également homogène (absence de protubérances ou de creux sur la surface libre);
  • une bonne adhérence de l'éponge sur son substrat, permettant d'extraire l'électrode de la cellule d'électrolyse dans risque de séparation ou de chûte de l'éponge.
Thanks to the process according to the invention, it is possible to produce cadmium sponges of very large thicknesses (typically 3 to 6 cm) which are characterized by:
  • good structural uniformity over the entire thickness;
  • an equally uniform development in thickness (absence of protuberances or hollows on the free surface);
  • good adhesion of the sponge to its substrate, making it possible to extract the electrode from the electrolysis cell at risk of separation or of falling of the sponge.

La séparation de l'éponge de son substrat (débatissage) s'effectue par des moyens mécaniques légers de type classique.The separation of the sponge from its substrate (stripping) is carried out by light mechanical means of conventional type.

L'éponge séparée est alors lavée afin de récupérer l'électrolyte qui l'imprègne encore. Ici encore, la structure particulière de l'éponge permet un lavage très efficace avec une quantité d'eau très faible.The separated sponge is then washed in order to recover the electrolyte which still permeates it. Here again, the special structure of the sponge allows very effective washing with a very small amount of water.

Une fois lavée, l'éponge s'avère parfaitement stable chimiquement, que ce soit vis-à-vis d'une dissolution par attaque acide ou par oxydation à l'air.Once washed, the sponge proves to be perfectly chemically stable, either with respect to dissolution by acid attack or by air oxidation.

La seconde opération du procédé consiste à effectuer une dilacération de l'éponge. Elle est réalisée dans des appareils de délitage à cuve munis de mobiles d'agitation particuliers, fonctionnant en continu ou en discontinu. Afin de favoriser une libération complète des particules de l'éponge, il est ici préférable de travailler avec un taux de pulpe quine dépasse pas 200 g de matière sèche par litre. Comme on le verra plus loin, la vitesse périphérique du mobile d'agitation est un facteur important pour obtenir une granulométrie appropriée.The second operation of the process consists in performing a shredding of the sponge. It is carried out in tank disintegration devices provided with specific stirring mobiles, operating continuously or discontinuously. In order to promote a complete release of particles of the sponge, it is preferable here to work with a pulp rate that does not exceed 200 g of dry matter per liter. As will be seen below, the peripheral speed of the agitation mobile is an important factor in obtaining an appropriate particle size.

Après l'opération de délitage, on prévoit avantagueusement, mais facultativement, une opération de tamisage destinée à éliminer les particules grossières, et préférentiellement les particules de taille supérieure à 125 µm.After the disintegration operation, a sieving operation intended to remove coarse particles, and preferably particles larger than 125 μm, is advantageously provided, but optionally.

Une autre caractéristique importante de la présente invention réside en ce que l'étape de délitage libère des particules dont la taille et la solidité sont déterminées essentiellement par les conditions opératoires de l'étape électrolytique de constitution de l'éponge, et ne sont que très faiblement influencées par un temps de séjour trop long de la matière dans l'appareil de délitage et par le choix de la géométrie des mobiles d'agitation. Ainsi plusieurs types de mobiles de dilacération ont été testés, avec ou sans contre-pales, sans que soient modifiées de façon notable la morphologie et la granulométrie de la poudre. En outre on n'a observé aucun surbroyage de la poudre.Another important characteristic of the present invention resides in that the disintegration stage releases particles whose size and solidity are determined essentially by the operating conditions of the electrolytic stage of constitution of the sponge, and are only very weakly influenced by a too long residence time of the material in the disintegration apparatus and by the choice of the geometry of the stirring mobiles. Several types of shredding machines have therefore been tested, with or without counter-blades, without significantly modifying the morphology and particle size of the powder. In addition, no overgrinding of the powder was observed.

On a constaté par ailleurs que la granulométrie de la poudre obtenue, évaluée par le taux de refus à l'étape de tamisage, n'était influencée que par la vitesse périphérique des mobiles de dilacération.It has also been found that the particle size of the powder obtained, evaluated by the rejection rate at the sieving step, was only influenced by the peripheral speed of the grinding mobiles.

La bonne tenue mécanique des particules constituant la pulpe autorise un stockage à l'état décanté sans modification de la distribution granulométrique. En outre, la pulpe obtenue en sortie du délitage peut être pompée, par exemple par une pompe centrifuge à vortex, sans subir d'altération granulométrique.The good mechanical strength of the particles constituting the pulp allows storage in the decanted state without modifying the particle size distribution. In addition, the pulp obtained at the end of the disintegration can be pumped, for example by a centrifugal vortex pump, without undergoing any particle size alteration.

Comme on le verra en détail plus loin, la morphologie de la poudre obtenue est caractéristique du procédé selon l'invention. Les particules ont la forme de fougères constituées d'une colonne centrale de laquelle partent, avec un angle de l'ordre de 60°, des fougères secondaires. La forme globale est généralement aciculaire, forme bien adaptée à l'application visée.As will be seen in detail below, the morphology of the powder obtained is characteristic of the process according to the invention. The particles have the form of ferns made up of a central column from which leave, with an angle of the order of 60 °, secondary ferns. The overall shape is generally acicular, a shape well suited to the intended application.

On va maintenant décrire plus en détail la mise en oeuvre concrète du procédé selon l'invention.We will now describe in more detail the concrete implementation of the method according to the invention.

La cellule d'électrolyse peut être alimentée indiférremment par une solution pure de cadmium ou par du cadmium métallique de pureté appropriée.The electrolysis cell can be supplied either with a pure cadmium solution or with metallic cadmium of appropriate purity.

Dans le cas d'une solution de cadmium, on choisit de préférence une solution concentrée. L'anion associé est avantageusement le sulfate. L'acidité de la solution peut varier par exemple entre 5 et 80 g/l d'acide sulfurique. Pour l'obtention d'une poudre de cadmium de pureté compatible avec l'application visée, la teneur totale en impuretés métallliques de la solution, exprimée par rapport au cadmium, doit être inférieure à 100 g/t.In the case of a cadmium solution, a concentrated solution is preferably chosen. The associated anion is advantageously sulfate. The acidity of the solution can vary for example between 5 and 80 g / l of sulfuric acid. To obtain a cadmium powder of purity compatible with the intended application, the total content of metallic impurities in the solution, expressed relative to cadmium, must be less than 100 g / t.

Dans le cas où la cellule d'électrolyse est alimentée par du cadmium métal, il peut prendre toute forme approprié, avec de préférence une pureté de 99,99% ou mieux. On peut utiliser une anode coulée ou alimentée en boules ou baguettes de métal. Les essais ont permis de constater que, quel que soit le type d'alimentation, la réaction anodique ne limite en rien le processus d'obtention de l'éponge de cadmium à la cathode.In the case where the electrolysis cell is powered by cadmium metal, it can take any suitable form, preferably with a purity of 99.99% or better. One can use an anode cast or supplied with metal balls or rods. The tests made it possible to note that, whatever the type of supply, the anodic reaction in no way limits the process of obtaining the cadmium sponge at the cathode.

L'électrolyte est composé de sulfate de cadmium et d'acide sulfurique. La teneur en acide est conditionnée par la recherche d'une bonne conductivité ionique de l'électrolyte. Cette teneur est avantagueusement comprise entre 5 et 100 g/l, une valeur proche de 50 g/l étant particulièrement intéressante car elle confère urne très bonne conductivité tout en limitant la corrosion acide de l'éponge.The electrolyte is composed of cadmium sulfate and sulfuric acid. The acid content is conditioned by the search for a good ionic conductivity of the electrolyte. This content is advantageously between 5 and 100 g / l, a value close to 50 g / l being particularly advantageous because it gives a very good conductivity while limiting the acid corrosion of the sponge.

Le choix de la concentration en cadmium est étroitement lié au choix de la densité de courant cathodique. Des essais effectués par la Demanderesse ont permis de découvrir que, dans la gamme de densités de courant allant d'environ 700 à environ 1500 A/m², il est avantageux que la concentration en cadmium réponde à la relation suivante: 100 A.m/kg ≦ J/(CD) ≦ 200 A.m/kp

Figure imgb0001
où
J est la densité de courant exprimée en A/m²;
(CD) est la concentration en cadmium exprimée en kg/m³.The choice of the cadmium concentration is closely linked to the choice of the cathodic current density. Tests carried out by the Applicant have made it possible to discover that, in the range of current densities ranging from approximately 700 to approximately 1500 A / m², it is advantageous for the cadmium concentration to respond to the following relationship: 100 Am / kg ≦ J / (CD) ≦ 200 Am / kp
Figure imgb0001
 or
J is the current density expressed in A / m²;
(CD) is the cadmium concentration expressed in kg / m³.

Ainsi, pour une densité de courant comprise entre 900 et 1200 A/m², la concentration en cadmium est de préférence comprise entre 4 et 15 g/l, plus préférentiellement entre 7 et 11 g/l.Thus, for a current density of between 900 and 1200 A / m², the cadmium concentration is preferably between 4 and 15 g / l, more preferably between 7 and 11 g / l.

La température de fonctionnement est maintenue de préférence dans un intervalle compris entre 20 et 35°C, plus préférentiellement entre 25 et 30°C.The operating temperature is preferably maintained in a range between 20 and 35 ° C, more preferably between 25 and 30 ° C.

Comme on l'a indiqué, le substrat cathodique est de préférence de l'acier inoxydable ou du titane. On a constaté qu'une bonne adhérence de l'éponge était obtenue avec une rugosité de surface correspondant à l'état brut de laminage.As indicated, the cathode substrate is preferably stainless steel or titanium. It was found that good adhesion of the sponge was obtained with a surface roughness corresponding to the raw rolling state.

La circulation de l'électrolyte est assurée soit naturellement quant l'oxygène est dégagé, pour une cellule à anodes insolubles, soit de façon provoquée. Le choix du type de circulation n'a pratiquement aucune influence sur la morphologie de l'éponge.The circulation of the electrolyte is ensured either naturally when the oxygen is released, for a cell with insoluble anodes, or in a provoked way. The choice of circulation type has practically no influence on the morphology of the sponge.

La durée d'électrolyse entre deux débatissages est de préférence comprise entre 4 et 8 heures. Dans les conditions optimisées de densité de courant et de concentration en cadmium telle que mentionnées plus haut, urne durée de l'ordre de 6 heures convient tout particulièrement.The duration of electrolysis between two debates is preferably between 4 and 8 hours. In the optimized conditions of current density and cadmium concentration as mentioned above, a duration of the order of 6 hours is particularly suitable.

La conception concrète des cellules d'électrolyse est de type classique, et ne sera pas décrite en détail. On peut faire appel par exemple à des cellules du type utilisé dans l'industrie du zinc ou du cuivre.The concrete design of the electrolysis cells is of the conventional type, and will not be described in detail. It is possible, for example, to use cells of the type used in the zinc or copper industry.

Dans le cas d'un processus d'électrolyse à anodes solubles, on observe que la composition de l'électrolyte ne reste pas stable. En effet, la réaction à la cathode, où les protons sont réduits et de l'hydrogène est engendré, constitue une réaction parasite qui provoque une décroissance de l'acidité du milieu, à laquelle est associé un accroissement de la concentration en cadmium.In the case of an electrolysis process with soluble anodes, it is observed that the composition of the electrolyte does not remain stable. Indeed, the reaction at the cathode, where the protons are reduced and hydrogen is generated, constitutes a parasitic reaction which causes a decrease in the acidity of the medium, with which is associated an increase in the cadmium concentration.

Selon un aspect particulier de la présente invention, pour ne pas avoir à purger le système en cadmium et à ajouter de l'acide, on prévoit de combiner le processus d'alectrolyse à anodes solubles avec un processus d'électrolyse à anodes insolubles, travaillant sur le même électrolyte. Par simple ajustement de la surface cathodique du processus fonctionnant avec anodes insolubles, de telle sorte que cette surface constitue un pourcentage déterminé de la surface cathodique totale, pourcentage égal au rendement faradique cathodique de dégagement d'hydrogène, on compense alors précisément l'excès de dissolution anodique précité. On engendre également sur les anodes insolubles l'acidité consommée par la réaction parasite sus-mentionnée. Le système est donc globalement équilibré et peut fonctionner dans des conditions stables pratiquement sans nécessiter d'ajout ou de purge, ce qui garantit pour l'éponge formée, et par suite pour la poudre, une qualité constante.According to a particular aspect of the present invention, in order not to have to purge the cadmium system and to add acid, provision is made to combine the electrolysis process with soluble anodes with an electrolysis process with insoluble anodes, working on the same electrolyte. By simple adjustment of the cathodic surface of the process operating with insoluble anodes, so that this surface constitutes a determined percentage of the total cathodic surface, percentage equal to the faradic cathodic yield of hydrogen release, the excess of aforementioned anodic dissolution. The acidity consumed by the above-mentioned parasitic reaction is also generated on the insoluble anodes. The system is therefore generally balanced and can operate under stable conditions practically without the need for addition or purging, which guarantees constant quality for the sponge formed, and consequently for the powder.

Après électrolyse puis débatissage et lavage de l'éponge comme indiqué plus haut, l'éponge est soumise à l'opération de délitage. L'action de dilacération est réalisée par un mobile d'agitation n'ayant pas de fonction de pompage ou de cisaillement importante. On recherche en effet principalement un effet de choc sur les parties périphériques du mobile qui ont une surface active faible.After electrolysis then deburring and washing of the sponge as indicated above, the sponge is subjected to the disintegration operation. The shredding action is carried out by a stirring mobile having no significant pumping or shearing function. We are mainly looking for a shock effect on the peripheral parts of the mobile which have a small active surface.

Comme évoqué plus haut, le paramètre essentiel est la vitesse périphérique du mobile. Elle est de préférence située entre 20 et 50 m/s pour des diamètres de mobiles d'agitation variant entre 83 et 380 mm. Pour des vitesses inférieures à cette gamme, on observe un accroissement rapide du taux de particules refusées au tamisage. Concrètement, on a constaté que, pour un mobile d'un diamètre de 380 mm, une vitesse périphérique de 30 m/s était suffisante pour atteindre un taux de refus au tamisage à 125 µm inférieur à 0,5%.As mentioned above, the essential parameter is the peripheral speed of the mobile. It is preferably situated between 20 and 50 m / s for diameters of stirring mobiles varying between 83 and 380 mm. For speeds below this range, there is a rapid increase in the rate of particles refused to sieving. Concretely, it was found that, for a mobile with a diameter of 380 mm, a peripheral speed of 30 m / s was sufficient to reach a rejection rate at sieving at 125 µm of less than 0.5%.

Le temps de séjour des éponges dans l'appareil de délitage est par exemple compris entre 3 et 5 minutes. On a cependant constaté qu'un excès de temps de séjour de 100 à 200% par rapport à ces durées n'entraînait aucune conséquence sur la distribution granulométrique.The residence time of the sponges in the disintegration apparatus is for example between 3 and 5 minutes. However, it has been found that an excess of residence time of 100 to 200% over these durations has no consequence on the particle size distribution.

Le taux de pulpe est fixé à une valeur compatible à la fois avec les impératifs de productivité du procédé et avec l'impératif de conservation de la distribution granulométrique. Concrètement un quantité de matière sèche par litre de solution de mise en pulpe comprise entre 50 et 200 g/l s'avère appropriée. Au delà de la limite supérieure, la distribution granulométrique devient plus grossière.The pulp rate is fixed at a value compatible both with the productivity requirements of the process and with the conservation requirement of the particle size distribution. Concretely, an amount of dry matter per liter of pulping solution between 50 and 200 g / l is suitable. Beyond the upper limit, the particle size distribution becomes coarser.

Après l'opération de délitage, la pulpe est tamisée, comme indiqué plus haut, par exemple à l'aide d'un tamis vibrant. La pulpe est ensuite décantée et conditionnée. On a constaté dans des conditions de stockage humide un taux d'oxydation inférieur à 1% par mois.After the disintegration operation, the pulp is sieved, as indicated above, for example using a vibrating sieve. The pulp is then decanted and conditioned. An oxidation rate of less than 1% per month has been observed under wet storage conditions.

On a illustré sur les vues au microscope des figures 1 et 2 l'aspect de la poudre de cadmium obtenue. Les grossissements utilisés étaient respectivement de 200 et de 800. On observe une poudre dendritique dont les particules sont en forme de fougères caractérisées par une âme centrale, d'une section transversale comprise entre environ 4 et environ 20 µm², sur laquelle se sont développées des dendrites secondaires orientées obliquement par rapport à la direction de l'âme, avec une inclinaison moyenne d'environ 60°. L'aire spécifique de la poudre, mesurée selon la méthode BET, est comprise entre 1 et 3 m²/gu. Le diamètre moyen, déterminé par granulométrie au laser, est d'environ 20 µm, une répartition ganulométrique typique étant la suivante: d₉₈ ≈ 64 µm

Figure imgb0002
d₉₀ ≈ 37 µm
Figure imgb0003
 d₁₀ ≈ 7 µm
Figure imgb0004
 Illustrated on the microscope views of Figures 1 and 2 the appearance of the cadmium powder obtained. The magnifications used were 200 and 800 respectively. A dendritic powder is observed, the particles of which are in the form of ferns characterized by a central core, with a cross section of between approximately 4 and approximately 20 μm², on which secondary dendrites have developed oriented obliquely to the direction of the core, with an average inclination of approximately 60 °. The specific area of the powder, measured according to the BET method, is between 1 and 3 m² / gu. The average diameter, determined by laser granulometry, is approximately 20 μm, a typical ganulometric distribution being as follows: d₉₈ ≈ 64 µm
Figure imgb0002
 d₉₀ ≈ 37 µm
Figure imgb0003
 d₁₀ ≈ 7 µm
Figure imgb0004

Par ailleurs, le procédé de l'invention garantit un titre très élevé en cadmium métallique par rapport au cadmium total. Ainsi on constate que, malgré une surface spécifique très élevée, le produit final est très peu oxydé.Furthermore, the process of the invention guarantees a very high titer in metallic cadmium compared to the total cadmium. Thus, it can be seen that, despite a very high specific surface, the final product is very little oxidized.

Une composition typique est la suivante:

Cadmium total
≧ 99%
Cadmium métal
≧ 95%
Zn
≦ 50 g/t
Pb
≦ 30 g/t
Ni
≦ 10 g/t
Cu
≦ 10 g/t
Fe
≦ 30 g/t
SO₄=
≦ 50 g/t
A typical composition is as follows:
Cadmium total
≧ 99%
Cadmium metal
≧ 95%
Zn
≦ 50 g / t
Pb
≦ 30 g / t
Or
≦ 10 g / t
Cu
≦ 10 g / t
Fe
≦ 30 g / t
SO₄ =
≦ 50 g / t

Bien entendu, la présente invention n'est nullement limitée à la description ci-dessus, mais l'homme de l'art saura y apporter toute variante ou modification conforme à son esprit.Of course, the present invention is in no way limited to the above description, but those skilled in the art will be able to make any variant or modification in accordance with his spirit.

Claims (16)

Procédé d'obtention d'une poudre fine de cadmium dendritique, caractérisé en ce qu'il comprend les étapes suivantes: (a) production électrolytique de cadmium métal sur une électrode, dans des conditions telles qu'on forme une éponge constituée de dendrites polymorphes enchevêtrées, (b) débatissage et lavage de l'éponge, (c) délitage de l'éponge en milieu pulpeux dans des conditions telles qu'on libère les dendrites, pour obtenir une poudre dendritique de granulométrie essentiellement inférieure à une limite déterminée. Process for obtaining a fine powder of cadmium dendritic, characterized in that it comprises the following stages: (a) electrolytic production of cadmium metal on an electrode, under conditions such that a sponge made up of entangled polymorphic dendrites is formed, (b) stripping and washing the sponge, (c) disintegration of the sponge in pulpy medium under conditions such that the dendrites are released, in order to obtain a dendritic powder with a particle size essentially below a given limit. Procédé selon la revendication 1, caractérisé en ce que l'étape (a) est réalisée avec une densité de courant comprise entre 700 et 1500 A/m², une concentration en cadmium dans l'électrolyte comprise entre 3,5 et 15 kg/m³, et avec une solution électrolytique composée de sulfate de cadmium et d'acide sulfurique, la teneur en acide sulfurique étant comprise entre 5 et 100 g/l.Method according to claim 1, characterized in that step (a) is carried out with a current density of between 700 and 1500 A / m², a concentration of cadmium in the electrolyte of between 3.5 and 15 kg / m³ , and with an electrolytic solution composed of cadmium sulphate and sulfuric acid, the sulfuric acid content being between 5 and 100 g / l. Procédé selon la revendication 2, caractérisé en ce que la concentration en cadmium dans l'électrolyte est déterminée par la relation: 100 A.m/kg ≦ J/(CD) ≦ 200 A.m/kg
Figure imgb0005
 où
J est la densité de courant exprimée en A/m²;
(CD) est la concentration en cadmium exprimée en kg/m³.Method according to claim 2, characterized in that the concentration of cadmium in the electrolyte is determined by the relation: 100 Am / kg ≦ J / (CD) ≦ 200 Am / kg
Figure imgb0005
 or
J is the current density expressed in A / m²;
(CD) is the cadmium concentration expressed in kg / m³. Procédé selon l'une des revendications 2 et 3, caractérisé en ce que l'étape (a) est conduite à une température comprise entre 20 et 35°C, de préférence entre 25 et 30°C.Method according to one of claims 2 and 3, characterized in that step (a) is carried out at a temperature between 20 and 35 ° C, preferably between 25 and 30 ° C. Procédé selon l'une des revendications 2 à 4, caractérisé en ce que l'étape (c) est effectuée dans une cuve de délitage avec un taux de pulpe compris entre environ 50 et environ 200 g de cadmium par litre de solution de mise en pulpe et avec un mobile d'agitation dont la surface active vis-à-vis de la pulpe est faible et dont la vitesse périphérique est comprise entre environ 20 et environ 50 m/s.Method according to one of claims 2 to 4, characterized in that step (c) is carried out in a disintegration tank with a pulp content of between approximately 50 and approximately 200 g of cadmium per liter of pulping solution and with a stirring mobile whose active surface vis-à-vis the pulp is low and whose peripheral speed is between about 20 and about 50 m / s. Procédé selon la revendication 5, caractérisé en ce que l'étape (c) est conduite pendant une durée supérieure à trois minutes.Method according to claim 5, characterized in that step (c) is carried out for a period greater than three minutes. Procédé selon l'une des revendications 1 à 6, caractérisé en ce que l'étape (a) est réalisée en alimentant un appareil d'électrolyse avec du cadmium métal et en ce que l'appareil comprend au moins une cellule fonctionnant avec anodes solubles et au moins une cellule fonctionnant avec anodes insolubles, la surface cathodique de la ou des cellules fonctionnant avec anodes insolubles étant choisie de manière à constituer un pourcentage de la surface cathodique totale égal au rendement faradique cathodique de dégagement d'hydrogène.Method according to one of claims 1 to 6, characterized in that step (a) is carried out by supplying an electrolysis apparatus with cadmium metal and in that the apparatus comprises at least one cell operating with soluble anodes and at least one cell operating with insoluble anodes, the cathode surface of the cell or cells operating with insoluble anodes being chosen so as to constitute a percentage of the total cathode surface equal to the cathode faradic yield of hydrogen evolution. Procédé selon l'une des revendications 1 à 7, caractérisé en ce que l'étape (c) est suivie d'une étape (d) de tamisage de la pulpe pour éliminer les particules solides d'une taille supérieure à une taille limite déterminée.Method according to one of claims 1 to 7, characterized in that step (c) is followed by a step (d) of sieving the pulp to remove solid particles of a size greater than a predetermined size limit . Procédé selon la revendication 8, caractérisé en ce que l'étape (d) élimine les particules d'une taille supérieure à environ 125 µm.Method according to claim 8, characterized in that step (d) removes particles larger than about 125 µm. Poudre dendritique obtenue par le procédé selon l'une des revendications précédentes, caractérisée en ce que ses particules présentent la forme de fougères comportant une âme centrale de laquelle partent en oblique des dendrites secondaires.Dendritic powder obtained by the process according to one of the preceding claims, characterized in that its particles have the form of ferns comprising a central core from which oblique leave secondary dendrites. Poudre selon la revendication 10, caractérisée en ce que la section de l'âme centrale est comprise entre environ 4 et environ 20 µm².Powder according to claim 10, characterized in that the section of the central core is between approximately 4 and approximately 20 µm². Poudre selon l'une des revendications 10 et 11, caractérisée en ce qu'elle présente une surface spécifique, mesurée selon la méthode BET, comprise entre environ 1 et environ 3 m²/g.Powder according to either of Claims 10 and 11, characterized in that it has a surface specific, measured according to the BET method, between approximately 1 and approximately 3 m² / g. Poudre selon l'une des revendications 10 à 12, caractérisée en ce qu'elle présente une granulométrie inférieure à 125 µm.Powder according to one of claims 10 to 12, characterized in that it has a particle size less than 125 µm. Poudre selon la revendication 13, caractérisée en ce qu'elle présente un d₉₀ de l'ordre de 35 µm et un d₁₀ de l'ordre de 7 µm.Powder according to claim 13, characterized in that it has a d₉₀ of the order of 35 µm and a d₁₀ of the order of 7 µm. Poudre selon l'une des revendications 10 à 14, caractérisée en ce qu'elle comprend au moins 95% de cadmium métal.Powder according to one of claims 10 to 14, characterized in that it comprises at least 95% of cadmium metal. Poudre selon l'une des revendications 10 à 15, caractérisée en ce que l'angle moyen des dendrites secondaires par rapport à l'âme centrale est de l'ordre de 60°.Powder according to one of claims 10 to 15, characterized in that the average angle of the secondary dendrites with respect to the central core is of the order of 60 °.
EP92400200A 1991-01-28 1992-01-27 Process for manufacturing a fine dendritic cadmium powder and powder obtained by this process Expired - Lifetime EP0497675B1 (en)

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